Interesting....
I have never believed it necessary to change all eight when only one is bad.
It is only necessary that individuals in pairs be matched, then balance all
four of the push-pull amplifiers by way of adjusting the bias. Also be sure
also that the voltage on each amplifier is the same.
If you have one bad transistor , I would change it and its mate on that
amplifier. Doesn't make sense to change the other six.
73
Phil K4DPK
www.k4dpk.com
----- Original Message -----
From: "JACrux" <G3JAG@patents.freeserve.co.uk>
To: <tentec@contesting.com>
Sent: Monday, November 14, 2005 11:16 AM
Subject: [TenTec] Hercules II repair: 253 tuner relay
> The eight transistors are matched. Swapping just one or two out for
unmatched
> replacements could spell trouble.
>
> A note I added to figure 4-6 of my Herc manual says set the bias for an
idling
> current of 500 mA.
> In my particular case, another note, this time scribbled on the front
cover,
> (don't ask) says the base voltages are all 0.67 when the bias is set for
500
> mA.
>
> When in doubt, ask TT.
>
> Unfortunately they have not seen the intermittent problem I have on
10/12m.
> Sometimes there is normal operation, a good 500w out on either band. Then
> sometimes the power is way down on 12m and only up to 80/100w on 10m. This
is
> unpredictable, in the sense that I cannot make it happen. It just happens.
>
> Guesswork suggests a relay contact (K12/K13) or a so far unidentified bad
> joint, but if anyone else has seen this one and has a fix, please let me
> know.
> -------------------------------------------------------
> The vacuum relay in the 253 tuner is an expensive item. Easy to get one
and
> install it; OUCH to pay for it. But as it is not hot switched, I will try
> replacing mine (when I have to) with something a lot less expensive. There
are
> 12 v relays around with 7ms operating speeds, coil to contact insulation
of
> several kV, and a current handling capacity of around 8-12 amps AC. I
found
> some costing as little as $5 each, slightly better than $100 plus for a
> vacuum relay.
>
> A probably very rare relay problem I had in my 253 was that a back contact
on
> one of the relays (I think it was K3) on the bridge pad board did not
> actually close completely. It was intermittent or even open circuit. I
found
> the problem when I managed to get the relay cover off so I could operate
the
> thing by hand and watch the contacts close. I fixed it by tweaking the
blade
> very gently so the back contacts closed firmly. I then cleaned the
contacts to
> remove the product of any arcing. End of erratic operation and irregular
SWR
> detection.
>
> Yes, that was a rather crude fix, but it has worked for several years
without
> needing a new relay. I am too far from TT to take the thing back, although
I
> did just that once with the Herc II, during a business trip to TN. In
those
> days, no-one asked what was in my hand baggage. I was asked only once to
open
> my bag .... at Nashville on the way home. The lady took one look at the
top
> of the Herc and decided I should shut the bag and get on the plane.
> 73
> John G3JAG
>
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>
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